Portions of the following discussion of the nasal anatomy of domestic mammals are excerpted from R. Nickel et al., The Viscera of Domestic Animals, (2nd revised ed.), Springer-Verlag, New York, Hiedelberg, Berlin (1979), pp. 211-221. This is an excellent text on the comparative visceral anatomy of domestic mammals. As used herein, the terms "mammal" and "animal" are used synonomously and refer to non-human mammals.
The nasal anatomy of domestic animals is considerably different than that of a human. Unlike the human nose that projects distinctly from the face, in domestic animals, the nose is incorporated into the face and forms the large dorsal and lateral areas rostral to the eyes. The nostrils in the apex of the nose are the entry to the respiratory system of domestic mammals. Once passing through the nostrils, inspired air moves into the nasal cavities and continues through the nasopharynx, larynx, trachea and lungs.
At the apical entrance to the nose the nostrils are partitioned by the nasal septum to divide the nasal cavity into right and left halves. The caudal portion of the septum is bony, while rostrally the septum consists of cartilage which becomes progressively more flexible toward the apex.
The wall of the nose consists of skin externally and a middle supporting layer of bone caudally and cartilage rostrally. The nasal cavity is lined by a mucous membrane. The rostral bones forming the wall of the nose include the nasal, maxillary and incisive bones. The free borders of the nasal and incisive bone provide attachment for the cartilages which support the nostrils. The supporting bones and cartilages of the nose are associated with the nasal muscles that regulate the size of the nostrils.
The dorsal and ventral lateral nasal cartilages are formed by the widening of the rostral part of the nasal septum along its dorsal and ventral margins. In the horse, the ventral lateral nasal cartilage is small and may be absent. In many domestic animals, there is no lateral support for the soft tissue over the rostral nasal passage caudal to the nostril.
A further difference in the formation of the nasal cartilages of the horse is the presence of alar cartilages. The alar cartilages consist of a ventral comu and a dorsal lamina and support the nostrils dorsally, medially and ventrally. The lamina of the alar cartilage and the medial accessory cartilage support the nasal diverticulum, a blind pouch in the dorsal aspect of the nostril.
The muscles of the nose and upper lip act to dilate the nostrils. This is particularly noticeable during labored breathing. In the horse, these muscles are well developed and can transform the normally semilunar nostrils to become circular.
The dorsal lateral area of the rostral nasal cavity that is caudal to the alar cartilages in the nostrils of the horse includes a region of unsupported soft tissue which can be drawn into the nasal cavity during inspiration of air into the nasal passages. The nasal diverticulum of the horse is a part of the soft tissue structures of the horse which can be drawn into the nasal cavity. When the soft tissue is drawn in, it can narrow the nasal cavity and reduce the area for the intake of air, thus reducing the air movement into the nasal passages and ultimately to the lungs where the oxygen is transferred in the pulmonary aveoli. The physiological effects of reduced oxygen transfer at rest and during physical exertion are documented. Some experts have theorized that exercise induced pulumonary hemorrhage (EIPH) in performance horses is caused by asphyxia due to abnormal resistance of a closed or partially closed upper airway. The upper airway being defined as the region of the respiratory tract lying between the nostrils and the windpipe at the level of the first rib. Hence, the nasal passages are part of this region. Dr. Robert Cook, "EIPH or AIPE? A Tufts University Researcher suggests that bleeding is not caused by EIPH, but by asphyxia", The Equine Athlete, p. 22-23 (March/April 1997).
Devices for dilating the outer wall tissue of the nasal passages in humans have been described in, for example, U.S. Pat. Nos. 5,533,503; 5,546,929; 5,553,605; and RE 35,408. These devices, however, do not address the unique soft tissue structures and mechanical problems associated with providing support for the nasal tissues of nonhuman mammals, especially large performance mammals such as the horse and camel.
Accordingly, there is a need to reduce the detrimental effects of reduced air intake, or to enhance the physiological benefit of increased air intake, during physical exertion of domestic mammals. Specifically, there is a need to increase, or reduce the decrease of, nasal passage narrowing that can occur during breathing in domestic mammals, especially performance animals such as the horse, camel, and dog.